1. Field of the Invention
The present invention relates to a traction drive fluid. More particularly, the present invention is concerned with a traction drive fluid which has excellent wear resistance and excellent scoring resistance, is capable of suppressing surface damage such as peeling of a friction material and in particular, is well suited for use as a lubricating oil for a traction drive continuously variable transmission used in an automobile.
2. Description of the Related Arts
A traction drive fluid is employed in a traction drive apparatus (friction drive apparatus by rolling contact drive), for instance, continuously variable transmissions for an automobile, continuously variable transmissions for industrial use and the like. In particular, a traction drive continuously variable transmission used in an automobile is equipped with a starting/running device and a lock-up clutch. Accordingly, the traction drive fluid employed therein is called upon to have lubricating characteristics for lubricating portions of high surface pressure such as gears, roller-bearings and traction drive portions, for instance, to have lubricity and peel resistant durability for a clutch friction material further to wear resistance and scoring (seizure) resistance therefor.
In recent years, automobile transmissions show a tendency to be miniaturized, light-weighted and maximized in transmission capacity. This is also the case with a traction drive continuously variable transmission. The aforesaid technological tendency accelerates heat generation at lubricating portions and brings about an increase in thermal load for a lubricating oil used therein. As a result, the traction drive fluid used therein is called upon to have heat resistance and besides, a high lubricating performance and adaptability to materials for lubricating portions under the condition of an elevated temperature.
Such being the case, the aforesaid traction drive fluid is required to be imparted with friction characteristics for a friction material (wet clutch) and at the same time, wear resistance and scoring resistance, thus bringing about a trend towards the use of an additive having high reactivity with a metallic surface.
However, an additive having high reactivity with a metallic surface, that is, an active additive is highly active also with cellulose which is a principal component of a friction material. Thus the aforesaid active additive suffers from such disadvantages that accelerate cutoff of cellulose structure (cutoff of cellulose fibers in the form of rings and chains) and elution thereof in oil or the like, and are more prone to cause damage such as peeling to a friction material.
In such circumstances, there has eagerly been desired the development of a traction drive fluid capable of alleviating and suppressing the reaction with cellulose which reaction is responsible for peeling and the like of a friction material, without impairing its wear resistance and scoring resistance of a steel material element.
There has only been reported the working effect of a metal base detergent (calcium sulfonate having a total base number of 300 ) as a technique for alleviating and suppressing the peeling damage to a friction material (refer to preliminary prints for scientific lecture, Japan Automotive Engineering Society, 952, 9535297, 1995-5 ). Nevertheless, in the case of blending the above-mentioned metal base detergent dispersant, there is caused the problem of a fear of clogging the pores on the surface of a friction material, thereby deteriorating the lubricating characteristics and the like.